Submersible pumps are designed to remove liquids from tanks and sumps and to operate in a submerged condition. Submersible pumps typically rely on submergence for cooling of the motor. Running the motor exposed to air would result in overheating of the motor and its premature failure resulting in costly repairs and possibly flooding or lost production. Controls, which add expense and complexity to the installation, are often employed to assure that the liquid levels are not drawn down below motor height. However, in most cases it is desirous to the operators of these pumps to empty the contents of the tank or sump to the greatest extent possible. The added liquid inventory necessary to keep the motor submerged often represents cost due to unusable production, or in the case of chemical plants, hazardous materials that pose environmental risk.
Manufacturers have used a number of methods to allow submersible motors to operate unsubmerged without overheating of the motor. All of these methods either add unnecessary cost or are ineffective when handling liquids containing solids. Some manufacturers install submersible motors rated for a much higher horsepower than the application will require. This allows the motor to operate at a fraction of its load carrying capability and at a fraction of its full load temperature. If large enough, an oversized motor can run unsubmerged without overheating. Although effective, it is a costly solution both from the standpoint of the initial motor cost and from the fact that the motor, operating at a fraction of its full load power, is also operating at less than optimum efficiency.
Still other manufacturers have installed a cooling jacket onto the motor frame, through which a clean cooling media is circulated from an external source. This method has the advantages of allowing the motor to be sized for its rated load, and also allows the pump to operate in a solids ladened environment, but it has the inherent disadvantage of additional costs related to the jacketing and the circulation system for the cooling media. Other methods take a slip stream from the pumpage and use the pressure developed by the pump impeller to cool the motor. Methods that have used a slip stream from the pumpage have proven to be unsuitable for applications where solids and slurries are present because the jackets are susceptible to plugging from deposited solids.
Stahle U.S. Pat. No. 4,349,322 teaches a spiral groove in the sealing cover located in close proximity to the impeller to create a shearing action to reduce the size of solids within a solids bearing fluid stream passing between the impeller and the sealing cover. The inner radius of the solids reduction device delivers a reduced solids flow stream into a seepage collection channel that in turn is tangentially fed to a cooling jacket around the motor. It is a well known fact to those familiar with the art that the available pressure from a pump is reduced as a function of the diameter change from the outside diameter of the impeller to its axis. In relying on flow traveling from the impeller outside diameter to the area in the vicinity of the impeller hub, Stahle reduces the pressure available to supply the motor cooling jacket.
Submersible motor jackets have a relatively high volume compared to the annulus around the impeller hub. Entering the expanded area of the jacket causes the fluid velocity to be further reduced. This can cause heavier solids to precipitate out of solution and remain in the jacket. Over time the solids will accumulate in the jacket resulting in reduced cooling capacity and premature motor failure. Further disadvantages are that both the circumferential grooving used by Stahle for size reduction and the motor jacket are expensive to manufacture.
Ivans U.S. Pat. No. 4,134,711 teaches the use of a sparge ring around the motor to spray pumped media upon the motor to cool it. Ivans teaches an alternative to this in U.S. Pat. No. 4,488,852 where he describes nozzles arrayed around the motor to spray pumped media onto the motor to cool it. Both of these methods are ineffective when handling solids ladened liquids. Solids large enough to pass through the pump are large enough, in most cases, to plug the comparatively small openings of the nozzles or sparge ring. The nozzle system also has the additional disadvantage of being ineffective when the motor is only partially submerged such that the nozzles are still covered in liquid and most of the motor is exposed. Under this partially submerged condition the nozzle discharge becomes diffused by the surrounding liquid, does not effectively cool the exposed motor shell and results in overheating and subsequent failure of the motor.